There are two main methods of determining the refractive properties of an eye for the purpose of providing correction lenses. One of those methods is subjective and the other objective. The subjective method is that of the familiar phoropter, in which the subject is asked to view a test plate, while optical elements of different spherical power and different cylindrical correction are inserted in front of the subject's eye under test. The subject provides the optometrist with the subject's view of which lens or lens combination provides the best vision, and the optometrist converges on this best subjective lens prescription.
There are a number of objective methods including that of direct measurement of the aberrations arising in a wavefront emitted from a point of light focused onto the retina of the eye of the subject. The difference between the aberrated wavefront emitted from the eye and a planar undistorted wavefront is measured using a wavefront analysis system, such as that based on a Shack-Hartmann array. The output of such an instrument is a map of the refractive properties across the eye, which can be used to determine the form and strength of spectacle lenses for correction of the aberration measured. A number of instruments have been described in the prior art for performing this wavefront analysis.
There is a certain advantage in a combination instrument involving a phoropter measurement in combination with a wavefront analyzer. Such a combination instrument has been described in International Patent Application Number WO 2005/037090 for “Method and Device for Determining the Residual Defective Vision of a Patient” to Ingo Mueller-Vogt, hereby incorporated by reference in its entirety. However the instrument described therein has a number of disadvantages. The illumination laser source is deflected into the measurement beam path on the imaging side of the phoropter assembly, such that it passes through the phoropter elements on its path towards the subject's eye. In its path through the phoropter elements, part of the incident laser light may be reflected back towards the Shack-Hartmann detection array, and the intensity of this reflected light may be substantially larger than that of the weak reflection from the retina, making measurement of the retinal reflection difficult. Furthermore, the mode of the laser beam may be degraded in its path through the phoropter elements.
There therefore exists a need for a combination optical refraction measurement system which overcomes at least some of the disadvantages of prior art systems and methods.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.